Epoxy resins and epoxy resin compositions containing a curing agent for epoxy resins as an essential component have excellent physical properties such as high heat resistance and moisture resistance and hence are widely used for, for example, semiconductor sealing materials, electronic components such as printed circuit boards, the electronic component field, conductive adhesives such as conductive pastes, other adhesives, matrices for composite materials, coating materials, photoresist materials, and development materials.
In recent years, further enhancement of properties such as heat resistance, moisture resistance, and solder resistance has been demanded in such various applications, in particular, applications to advanced materials. In vehicle-mounted electronic devices that are particularly required to have high reliability, the installation position has been changed from a cabin to an engine compartment having a higher temperature than a cabin. In addition, reflowing treatment temperature has increased due to use of lead-free solder. Therefore, high heat resistant materials that have higher glass transition temperature and can endure a thermal delamination test (hereinafter, abbreviated as “T288 test”) have been demanded.
When epoxy resin compositions are used as materials for printed wiring boards, a flame retardant containing halogen such as bromine is added together with an antimony compound to impart flame retardancy to epoxy resin compositions. However, with efforts in terms of environment and safety in recent years, there has been a strong demand for the development of an environmentally friendly and safe method for making compositions have flame retardancy without using halogen-based flame retardants that may emit dioxins and without using antimony compounds that may cause cancer. In addition, in the field of materials for printed wiring boards, use of halogen-based flame retardants causes degradation of reliability of printed wiring boards left to stand at high temperature. Accordingly, halogen-free compositions have been highly demanded.
As for an epoxy resin composition that satisfies such required characteristics and has flame retardancy and heat resistance, for example, PTL 1 discloses a technique of using, as an epoxy resin material or a curing agent for epoxy resins, a phosphorus-containing bisphenol that is obtained as follows: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (hereinafter, abbreviated as “HCA”) is allowed to react with p-hydroxybenzaldehyde and then the reaction product is allowed to react with phenol.
However, such a phosphorus-containing bisphenol has extremely high crystallinity and exhibits almost no solubility in a solvent. Therefore, such a phosphorus-containing bisphenol cannot be prepared in the form of varnish for the printed wiring board materials, and a cured product obtained by using the phosphorus-containing bisphenol as a curing agent for epoxy resins does not have satisfactory flame retardancy. In addition, since the melting point of the phosphorus-containing bisphenol is 200° C. or more, it is extremely difficult to perform industrial production.
NPL 1 discloses a technique of producing an oligomer in THF from an intermediate product obtained through a reaction between HCA and p-hydroxybenzaldehyde.
However, in the technique disclosed in NPL 1, the reaction product of HCA and p-hydroxybenzaldehyde, which is an intermediate product, has extremely high crystallinity and thus has low solubility in a solvent. Therefore, as described in NPL 1, THF, which is a dangerous solvent having a low flash point, needs to be used in the subsequent reaction and thus it is impossible to perform industrial production. In addition, the obtained oligomer itself has low solubility in a solvent and thus it is difficult to prepare a varnish for printed wiring board materials.
Furthermore, PTL 2 discloses a technique of producing a phosphorus-containing phenolic compound through a reaction between HCA and hydroxybenzaldehyde. However, the phenolic compound disclosed in PTL 2 is a monofunctional phenolic compound and thus has extremely high crystallinity and low solubility in a solvent. In addition, even when the phenolic compound is used as a curing agent for epoxy resins, sufficient flame retardancy is not achieved.